1. Field of the Invention
The present invention relates to a process for producing a low polarization mode dispersion (PMD) optical fiber.
2. Description of the Related Art
Optical signals transmitted through single-mode optical fibers comprise two orthogonal polarization modes (typically denoted TE and TM) that, in case of a fiber with a perfectly cylindrical core of uniform diameter, propagate independently of one another at a common velocity. Real optical fibers have considerable variations in the shape of the core along their length. They may also undergo non-uniform stresses such that their cylindrical symmetry is disrupted. Because of these factors, a phase difference can accumulate between the two modes, and the fiber is said to show “birefringence”.
An important parameter in the study of the birefringence of an optical fiber is what is known as the beat length, which, as described in U.S. Pat. No. 5,418,881 in the name of AT&T Corp., corresponds to the length of fiber required for a given state of polarization to be repeated. In other words, the beat length corresponds to the length of fiber required for the two components of the fundamental mode, initially in phase with each other, come to be in phase again, on the assumption that the fiber maintains a constant birefringence over this length.
When pulsed signals are transmitted in an optical fiber, the birefringence is a potential cause of the spreading of the pulses. This is because, if an input pulse excites both of the polarization components, it becomes wider at the output of the fiber, since the two components are dispersed along the fiber owing to their different group velocities. This phenomenon, known as polarization mode dispersion (PMD), has been widely studied in recent years because of its importance in periodically amplified light guide systems.
Typically, the phenomenon of PMD leads to a limitation of the width of the signal transmission band and, consequently, a degradation of the performance of the optical fibers along which the aforesaid signals are transmitted. This phenomenon is therefore undesirable in systems of signal transmission along optical fibers, especially in those operating over long distances, in which it is necessary to minimize any form of attenuation or dispersion of the signals to guarantee high performance in transmission and reception.
The process of drawing an optical fiber is typically carried out by means of suitable equipment known as “drawing tower”, starting from a glass preform. The structural and geometrical irregularities of the optical fiber that give rise to birefringence mainly originate during the process of manufacturing of the glass preform. In the drawing process, after the preform has been placed in a vertical position and heated to a temperature above the softening point, the molten material is drawn downwards at a controlled velocity in such a way as to produce a threadlike element that forms the optical fiber itself.
A possible solution to the problem of PMD is to spin the fiber about its axis during drawing, so as to rotate its polarization axes. As a result, the optical pulse propagates alternately on the slow and on the fast axis, thus compensating the relative delay and reducing the pulse spreading. This is equivalent to have a local effective refractive index for the pulse equal to the mean refractive index on the two axes, the average being taken over the pulse length along the fiber. With “spin profile” or “spin function” it is intended the function correlating the number of turns per meter imparted to the fiber with the position along the fiber.
The effect of spin on PMD is also equivalent to a reduction of the effective fiber birefringence. The higher the rate of axes rotation (twist/m), the lower the local effective birefringence. So a practical parameter that allows evaluating the PMD reduction of a generic spin profile is the average of spin rate modulus. The highest is the natural birefringence of the fiber, the highest this parameter should be in order to guarantee a low PMD.
The study of the different possible spin functions has led to disregard unidirectional spin profiles, because of the difficulty in removing the elastic torsion transmitted along the fiber downstream the spin application point, which torsion is therefore collected in the fiber wound on the bobbin.
Bidirectional spin, by alternating the spin direction, can prevent accumulation of residual elastic twist in the fiber.
Several different bidirectional spin profiles have been proposed in the past.
U.S. Pat. No. 6,148,131 relates to a method of making a twisted optical fiber with low polarization mode dispersion method wherein the fiber is bidirectionally spun with a uniformly periodic function of longitudinal position.
International patent application WO 97/26221 illustrates different alternate spin profiles, such as sinusoidal, square and triangular, and suggests the use of a spin function that is not substantially sinusoidal and has a sufficient variability to provide a reduction in the PMD for a plurality of beat lengths, for example a frequency-modulated sinusoidal function or an amplitude-modulated sinusoidal function.
U.S. Pat. No. 5,298,047 teaches imparting to the fiber an alternate spin not having a constant spatial frequency, so that the resulting fiber has a variable spatial spin pitch.